(1) Field of the Invention
The invention relates to the general field of integrated circuits and more particularly to the problem of applying a liquid layer to the surface of a relatively large silicon wafer in reasonable time and with reasonable uniformity.
(2) Description of the Prior Art
In the course of manufacturing an integrated circuit, many solid layers get deposited, one on top of another. It is also necessary sometimes to deposit a liquid layer such as photoresist solution or a solution of an uncured spin-on-glass. A commonly used method for accomplishing this is to spin the integrated circuit about an axis perpendicular to its main surface and to then drip one or more drops of the liquid onto the surface. Centrifugal force then spreads the liquid out, producing a uniform layer. As the silicon wafers from which integrated circuits are fashioned grow larger, this method begins to run into difficulty. In particular, there is a tendency for the thickness of the liquid layer near the outer edges to be thinner than desired.
This problem has been largely solved by the method and apparatus of the present invention. Instead of dispensing the liquid onto the spinning surface from a single source, located above the center of rotation, multiple sources, spaced at varying distances from the center of rotation, are used.
An apparatus and method involving two nozzles dispensing liquid onto a rotating substrate has been described by A. Kawai and S. Uoya in U.S. Pat. No. 5,001,084 (Mar. 19, 1991) but it is key to their invention that the nozzles dispense different liquids. The nozzle located over the center of rotation is used to dispense the liquid that is desired as the end product while the second nozzle is used to dispense a cleaning fluid. To practice their invention, the cleaning fluid is first dispensed and, only after it is no longer present on the spinning surface, is the other liquid dispensed from the central nozzle. Thus, as far as thickness distribution is concerned, this method suffers the same disadvantages as the prior art.
It is also clear that U.S. Pat. No. 5,001,084 is limited to two nozzles or, at best, an even number of nozzles, a limitation to which the present invention is not subject.